The hypothesis of this study was that growth of Porphyromonas gingivalis can be regulated by low hemin concentrations, that hemin limited growth will affect putative virulence factors, and that P. gingivalis possesses a specific receptor(s) for hemin/CR which is environmentally regulated by the concentration of hemin. P. gingivalis strain A7A1-28 (ATCC 53977) was grown under high, low, and limiting hemin concentrations. Bacterial cells grown in continuous cultures were analyzed for changes in putative virulence factors and membrane protein profiles. Hemin and CR binding by P. gingivalis were determined using cells from static broth cultures. Hemin binding activity was measured with radiolabeled hemin and hemin derivatives, and with a spectrophotometric assay for CR. Binding data was analyzed using the LIGAND computer program. Growth of P. gingivalis A7A1-28 was limited by hemin concentrations below 0.1 micrograms g/.m1. cells grown under hemin limitation showed decreased activity of putative virulence factors such as cell associated proteases and hemagglutinin. Decreased virulence in the mouse abscess model is also seen under hemin limitation. Alterations in protein profiles of bacterial cells membranes occurred at low hemin concentrations. Both hemin inducible (HeIPs) and hemin repressible (HeRPs) proteins were seen. Serums from patients with periodontitis have antibodies which react with HeRPs. Hemin limitation was also associated with alterations in proteolytic processing of the hemagglutinating adhesin HA-Ag2. These results suggested that under very low hemin concentrations, such as that found in uninflammed gingiva, P. gingivalis may not exert a strong pathogenic effect. When hemin is available, P. gingivalis growth and expression of virulence factors increases, which may lead to tissue destruction. This result is consistent with a model of periodontitis which postulates intermittent disease activity.